Electroplating baths for the deposition of gold-tin alloy have been found by the current inventors to be incapable of depositing the eutectic alloy over a usable current density range. This was clearly demonstrated in “Film growth characterization of pulse electro deposited Au/Sn tin films” by Djurfors and Ivey (GaAs MANTECH, 2001), where they show a step transition from 16 at % Sn to 50 at % at a current density of around 1.5 mA/cm2. According to the authors this is a result of the deposition of two distinct phases; Au5Sn (16 at % Sn) at low current density and AuSn (50 at % Sn) at high current density. This has been further confirmed by our work which has shown that prior art electrolytes will not typically yield the desired eutectic alloy.
The prior art electrolytes, using complexing agents such as citric acid, pyrophosphate, gluconic acid, ethylene diamine tetra acetic acid (“EDTA”), and the like, typically yield alloys which are either tin rich (<50% Au) or gold rich (95% Au), or have tin rich or gold rich regions at different current densities. An 80/20 wt % eutectic gold-tin alloy cannot be deposited over a usable current density range. Moreover, many prior art baths suffer from poor stability making them of little practical interest.
U.S. Pat. No. 4,634,505 by Kuhn, et al. describes an electrolyte using trivalent gold cyanide complex and a tin IV oxalate complex, which operates at pH below 3. The formulation also uses oxalic acid as a conducting salt. However, this bath gives deposits with less than 1% Sn, and therefore it is not useful for depositing a eutectic alloy.
U.S. Pat. No. 4,013,523 by Stevens et al. describes a bath using a trivalent gold complex and tin as a stannic halide complex. The pH is less than 3 and the bath is claimed to be capable of depositing an 80-20 wt % gold alloy.
U.S patent publication 2002063063-A1 by Uchida et at. describes a non-cyanide formulation where the gold complexes include gold chloride, gold sulfite and gold thiosulfate among others. The electrolyte includes stannic and stannous salts of sulfonic acids, sulfosuccinates, chlorides, sulfates, oxides and oxalates. The tin is complexed with EDTA, DTPA, NTA, IDA, IDP, HEDTA, citric acid, tartaric acid, gluconic acid, glucoheptonic acid among others. The deposit is brightened by a cationic macromolecular surfactant. Oxalate is listed among the possible buffer compounds.
Japanese patent application 56136994 describes a solution, which uses sulfite gold complex in combination with stannous tin pyrophosphate complex at a pH of 7 to 13.
German patent DE 4406434 uses trivalent gold cyanide complex in conjunction with stannic tin complexes. The pH is 3-14 and an 80-20 eutectic alloy is reported.
U.S. Pat. No. 6,245,208 by Ivey et al. discloses a non-cyanide formulation which uses gold chloride in combination with sodium sulfite, stannous tin, a complexing agent (ammonium citrate), and uses ascorbic acid to prevent oxidation of divalent tin. Eutectic alloy deposits are claimed and bath stability on the order of weeks is reported.
As noted, the prior art electrolytes are not always stable and have been found to be ineffective in providing eutectic gold tin alloy, particularly for electroplating of small parts for electronic components or composite substrates.
Accordingly, there is a need for a stable electroplating bath for the deposition of a eutectic gold-tin alloy on various substrates, and this is now provided by the present invention.